1. Field of the Invention
This invention relates to a fluid jet system and method for removing deposits, organic and inorganic, from submerged surfaces and for smoothing those surfaces. More particularly, this invention relates to an assembly of underwater, diver-controlled equipment, and the methods of using the same to provide rapid and efficient cleaning and smoothing of submerged surfaces, without harming either the surface material or protective coatings, in order to maintain ship performance.
2. Description of the Related Art
The degree of surface roughness of submerged portions of ships has a great effect on both ship fuel efficiency and the speed which can be achieved at a given propeller revolution rate. Roughness can be caused by marine growth ("fouling"), degradation of hull coatings, and deterioration of unpainted surfaces such as propeller blades. For commercial, private, or military ships, losses in ship performance can have a variety of consequences, both financial and in terms of meeting scheduled arrival dates.
Although the following examples are for a VLCC (Very Large Crude Carrier; an oil tanker, with the following typical approximate specification: 272,000 tons deadweight; total engine horsepower (at 90 RPM propeller rate): 32,700 hp), examples could be given for any size or type of marine craft. A typical trip for a VLCC is from the U.S. Gulf Coast to the eastern end of the Mediterranean Sea. This round trip normally takes about 40 days. However, with an increased surface roughness causing a loss in peak speed of only 1 knot (nautical mile per hour), 2 1/2 days would be added to the trip. At a typical $15,000 per day of lost utilization, this would cost the tanker owner about $37,500.
Considering the effect of surface roughness on efficiency, for a VLCC, each increase of 1 RPM in propeller rotation rate corresponds to an increase in ship speed of about 0.15 knot. Thus, a roughness caused loss of one knot would require an increase of about 6.7 RPM to maintain the same ship speed (i.e., to overcome the increased ship resistance). This increased propeller speed costs about 20 tons (metric ton) per day of extra fuel. At a cost of about $75 per ton, for the 40 day round trip discussed above, this would cost about $60,000.
Marine engineers estimate that an increase in the average roughness of a ship's hull of about 30 microns (peak-to-peak, RMS roughness) can cause a drop in peak achievable speed of about one percent. A new hull can have a surface roughness of about 160 micron. A deteriorating AF (Anti-Fouling) coating can be about 280 micron. This roughness increase could cause a four percent drop. For a typical 16 knot VLCC peak speed, this would be a loss of about 0.64 knots. Additional roughness due to a fouled propeller could easily double this speed loss.
The foregoing clearly demonstrates the economic importance of maintaining the submerged surfaces of ships in as smooth a condition as is practical. Therefore providing a means to maintain surface smoothness of ships is a practical and economical objective for ship owners.
The usual method of ship hull maintenance is to remove the ship from service, place it in a dry dock, and sandblast off the marine growth and all or part of the protective coating systems. Usually all of the AF (anti-fouling) coating is removed, and loosely adhered AC (anti-corroding) coating layers are also removed. The hull is inspected for damage or deterioration; repaired if necessary; and new AC and AF coatings applied. However, areas of about 3 by 8 ft., where the hull rests on the dock-blocks which support the VLCC in dry dock, are not coated. There can be as many as 400 "dock-block-shadows," and as much as ten percent of the hull can be involved. Because these "shadows" are not coated, they foul very rapidly once the VLCC is returned to service. As the typical period between dry docking for a VLCC is about 36 to 60 months, marine growth on the uncoated "shadows" can be 1 ft or more by the next dry docking. Therefore, the "shadows" are a major source of performance loss.
The dry docking process is very expensive and the ship is removed from useful service during dry docking. In addition, the AF coating can begin to lose its effectiveness after only 18 months of service.
It would therefore be desirable to provide a means to maintain hull and propeller smoothness by removing marine growth between dry dockings so that dry docking frequency can be decreased without performance loss.
Some methods for underwater removal of fouling from ship hulls and propellers have been used. For example, devices have been proposed which consist of one or more fixed or rotating brushes, configured in various ways and sizes; ranging from small, single brushes that a diver may use to clean a propeller, to a large powered brush system. An example of such a brush cleaner is U.S. Pat. No. 3,859,948 to Romano et al. However, these devices have a number of unsatisfactory characteristics.
The principal disadvantages of the powered, rotary brush systems, when used for hull cleaning, are:
(a) damage to AF coatings ---- The brushes score and roughen these soft coatings. The increased roughness due to coating damage can significantly offset the gains from fouling removal. Thus, such systems do not achieve the full potential objective of reducing surface roughness to reduce speed and energy losses.
(b) increase in the rate of subsequent marine growth ---- The brushes merely cut, and do not fully remove the stalks of marine plants. Thus, the remaining stalks bifurcate, and experience enhanced subsequent growth. The cut-free portions, on the other hand, are smeared around on the surface and are left to re-root. Similarly, seeds are disturbed and then re-implanted. By these three mechanisms, because rotary brushes do not fully remove and blast away the vegetative growths, the subsequent regrowth is faster than the pre-brushing growth rate. This requires more frequent brush cleanings in an attempt to maintain ship performance.
A variety of other surface cleaning devices have also been developed which use water jets, sand blasting nozzles, or brushes. Typical are the devices disclosed in U.S. Pat. Nos. 4,163,455, 4,220,170, and 4,462,328; and Japanese Patent No. 58-236285. Each of these devices, however, require some type of external means for: (i) causing the cleaning unit to adhere to the surface being cleaned; and (ii) causing the cleaning unit to be positioned and moved along that surface. Thus, because these devices lack an independent capability for performing these functions, they are incapable of effectively servicing the complex, varying surfaces, in terms of cleaning/smoothing requirement, represented by a submerged ship hull.
Yet another system which was developed for cleaning/smoothing the hulls of smaller, typically privately-owned boats, and some smaller commercial craft is illustrated in FIG. 18. This earlier cart used several, independent sets of water jet nozzles to perform the functions of forwardly propelling the cart, steering the cart, clamping the cart to the ship hull, and cleaning/smoothing the hull. In that design, then, water jets 70 provided forward propulsion, jets 73 disposed on each side of the cart near the front were intermittently activated by the diver to steer the cart, and a set of jets 72 on each side provided the force which clamped the cart wheels 26 to the ship hull. Thus, no hydraulic fluid powered motors were required for the cart's operation. Such a design was particularly well suited for the servicing of smaller, private boats, situated in crowded marinas, and where it is desirable to minimize the diesel engine noises and to avoid the chance of polluting the marina as a result of a hydraulic fluid leak. However, that design was unsuitable for cleaning the hulls of larger ships.
The type, location and extent of fouling on ship hulls determines what influence the fouling is having on ship performance. It would therefore be desirable to provide a method for surveying the underwater surfaces of the ship prior to initiation of a cleaning process so that a decision can be made as to whether an underwater maintenance effort is necessary or desirable to improve ship performance and to what parts of the hull should be cleaned. An approach to underwater hull inspection has been disclosed in U.S. Pat. No. 3,776,574. That approach calls for marking the hull with a visible subdividing, to indicate each discrete subarea on the hull; and marking a number or letter in each of these subdivided areas, thus providing a "map" for the diver to follow during his underwater inspection. It would be desirable, however, to provide an underwater hull inspection procedure which does not require such an artificial marking of the hull.